The present invention pertains to a process for regulating a turbocompressor to prevent surge in which a difference signal is generated from a continuously determined actual value of an operating variable of the compressor and a set point which depends on the position of the working point in the characteristic diagram and an input signal for the controller, which controller controls a valve branching off from the compressor outlet, is obtained from the difference signal using a time element, as well as to a device for carrying out the process.
The jerky or periodic backflow of medium being delivered from the delivery side to the intake side in compressors is called pumping or surge. This state occurs, e.g., if the end pressure is too high and/or the throughput is too low. A surge limit line, which separates a stable range of the characteristic diagram from an unstable range to the left of the surge limit line, can therefore be defined in the characteristic diagram. Operation in the unstable range to the left of the surge limit is not permissible, because severe damage to the machine may occur within a very short time. To avoid surge, i.e., the operation in the unstable range, a anti-surge controller is used, which controls a valve at the compressor outlet, which is connected as a anti-surge valve to the atmosphere or as a recycle valve to the intake side of the compressor. By opening the valve, the flow through the compressor is increased to the extent that the working point always remains within the stable range of the characteristic diagram. A control line (anti-surge or recycle curve) is defined for such a control in the characteristic diagram at a safety margin from the surge limit line. When the instantaneous working point is approaching the control line, the anti-surge or blow-by valve (hereinafter called anti-surge valve only) is more or less opened.
More precisely, such a control operates such that the set point for the flow control is determined from the compressor pressure or from the pressure ratio between the outlet pressure (end pressure) and the inlet pressure, or from a variable derived from this pressure ratio. This set point corresponds to the control line. The measured compressor intake flow is compared with the set point, and the anti-surge valve is adjusted in case of a deviation. If the working point of the compressor is on the control line, the control deviation of the surge limit controller is zero and the anti-surge valve remains in its position. If the working point exceeds the control line in the direction of the surge limit, the controller opens the valve wider, and if the working point is located to the right of the control line, the controller closes the valve.
During normal operation of the compressor, the working point of the compressor is markedly to the right of the control line (the design point is typically 20% to 30% to the right of this) and the anti-surge valve is completely closed. In case of a shift of the working point from this operating state in the direction of the surge limit, a conventional controller begins to open only when the actual value drops below the set point, i.e., when the working point has exceeded the control line in the direction of the surge limit.
Processes of the aforementioned kind for controlling a turbocompressor to prevent surging are described in the present Inventor""s earlier U.S. Pat. Nos. 4,298,310, 4,789,298, 4,810,163 and 4,968,215, the entire contents of which are incorporated herein by way of reference.
The aforementioned U.S. Pat. No. 4,298,310 discloses a process in which a difference signal is generated from a continuously determined actual value of an operating variable of the compressor and a set point, which depends on the position of the working point in the characteristic diagram and an input signal for the controller, which controller controls a valve branching off from the compressor outlet, is obtained from the difference signal using a time element. In this prior-art process, the difference signal from the set point and the actual value is sent once without delay and, in parallel thereto, with a delay to a subtraction point, from which the input signal is taken for the controller. This has the advantage that the control circuit can also process rapid, transient changes of the working point with sufficient reliability. The effect of the system is that an additional signal, which causes such a shift of the control line during transient working point shifts that when the working point approaches the control line, the safety margin between the surge limit and the control line is increased and the controller responds sooner as a result, is added to the set point of the controller. The control line is shifted quasi dynamically and a new xe2x80x9cdynamic control linexe2x80x9d is in effect. The consequence of this is that the safety margin between the control line and the stability limit is markedly greater under transient conditions than under steady-state conditions and the compressor is protected considerably better under such critical conditions.
However, the prior-art process has the drawback that even though the safety margin is increased during transient working point shifts which take place from a steady state in the direction of the surge limit, the controller can follow changes with a delay only, with the time constant set on the time element. The prior-art process is fully effective only when the working point is shifted in the direction of the surge limit from a steady-state working point. By contrast, the prior-art process works only unsatisfactorily in the case of disturbances that lead first to a shifting of the working point away from the surge limit and then again in the direction of the surge limit. When the working point is moving away from the surge limit, the control line is first shifted transiently to the left, with the tendency of being again set at the steady-state value according to a set time constant, i.e., normally over several minutes. A new steady state can be assumed and the prior-art process can show its full effectiveness only after this state has subsided. Until the subsidence of this transient state, the dynamic control line (this is the effective control line) is located to the left of the steady-state control line. The surge limit controller therefore interferes only with the delay, because the working point must be transiently shifted farther in the direction of the surge limit until the controller comes into action.
The basic object of the present invention is to improve a process and a device of the prior-art type such that the advantage of the increase in the safety margin can always be utilized to the full extent, regardless of whether the working point is located before the beginning of the disturbance in a steady operating state or whether transient working point shifts had already taken place before.
According to the invention, a process is provided for regulating a turbocompressor to prevent surge. A difference signal is generated from a continuously determined actual value of an operating variable of the compressor. A set point, which depends on the position of the working point in the characteristic diagram and an input signal for the controller, which controller controls a said valve branching off from the compressor outlet, is obtained from the difference signal using a time element. The difference signal is delayed with different time constants, depending on the direction in which it changes (increase or decrease). The controller responds more slowly to working point shifts in the direction of the surge limit line and more rapidly to working point shifts in the opposite direction.
The invention also provides a device for regulating a turbocompressor to prevent surge, with a measuring transducers for determining the actual value of one or more operating variables characteristic of the working point of the said compressor. A set point transducer is provided with a preset course and a control line in the characteristic diagram of the compressor A difference member is provided for generating a difference signal from the set point and the actual value. A controller generates a control signal for a valve at the compressor outlet. A circuit is provided for generating the input signal for the controller, which circuit contains a time element and to which the difference signal is sent. The time element is an asymmetric time element, whose delay is greater during a change in the difference signal that corresponds to a shift of the working point in the direction of the surge limit line than during a change in the difference signal in the opposite direction.
The time constant of the time element is asymmetric according to the present invention In case of working point shifts in the direction of the surge limit, the time element acts as described in the state of the art. In case of a shift of the working point in the direction of away from the limit line, the time element operates, by contrast, with a markedly smaller time constant. It is guaranteed as a result that the control line follows nearly without a delay in the case of working point shifts away from the boundary line, but with a known, markedly slower time constant in case of a shift in the direction of the surge limit.
In other words, the prior-art surge limit control for turbocompressors with control line set stationarily, which acts at a fixed distance to the right of the surge limit, is expanded according to the present invention with a dynamic control line. This dynamic control line is implemented such that it changes the effective position of the control line during transient shifts of the compressor working point in the direction of the surge limit, doing so such that depending on the rate at which the working point approaches the surge limit, the effective control line is shifted to the right in the characteristic diagram in the direction of the working point, with the consequence that the safety margin between the surge limit and the control line is increased and the surge limit controller will act sooner as a consequence. In case of very rapid shifts of the working point in the direction of the surge limit, the control line is shifted to the right by half the distance between the working point and the steady-state control line, and the shift of the control line is smaller in the case of slower shifts of the working point. If the working point is shifted in the direction of the steady-state control line only very slowly, i.e., over, e.g., 1 hour, the dynamic control line will coincide almost completely with the steady-state control line.
The regulation process according to the present invention is particularly suitable for applications in which a controlled variable, especially the flow signal, is very noisy due to flow whirl at the measuring point. Classical PID (proportional integral derivative) controllers with differentiating algorithms fail in these applications, because the differential component responds to the rate of change of the controlled variable. Differentiating control algorithms are unacceptable in the case of high-frequency signal disturbances (with frequencies of a few Hz) with signal deviations of a few percent, because they lead to considerable changes in the signal of the manipulated variable even during steady-state operation of the machine. They would respond in the vicinity of the design point even during steady-state operation and often prevent the complete closing of the valve during steady-state operation to the right of the control line. However, the valve is to be kept completely closed during steady-state operation for economic reasons. The process according to the present invention offers marked advantages here, because it does not show this disadvantageous effect even in the case of extremely noisy controlled variables.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.